1. Field of the Invention
The present invention relates to an engine idle speed control system for an automotive vehicle provided with an air conditioner.
2. Description of the Prior Art
An air conditioning apparatus such that dehumidification performance can be improved at low external temperatures below a predetermined value is well known, as disclosed in Japanese Published Unexamined (Kokai) Patent Application No. 1-254418.
The above-mentioned control system is referred to as constant temperature demist control. In this constant temperature demist control in an air conditioning apparatus using a compressor of fixed capacity type as shown in FIG. 6, the compressor is deactuated when the temperature T.sub.INT of air having passed through an evaporator (referred to as after-evaporator temperature, hereinafter) drops down to a low temperature (e.g., 3.degree. C. at which the evaporator starts to be frozen) (at the time t.sub.1) and actuated again when the after-evaporator temperature T.sub.INT rises up to another higher temperature (e.g. 4.5.degree. C.) (at the time t.sub.2) That is, the dehumidification performance can be improved at low temperatures by actuating the compressor near the utmost possible limit where the evaporator is not frozen.
On the other hand, in an automotive vehicle on which the air conditioning apparatus as described above is mounted, an auxiliary air control valve (referred to as AAC valve) for controlling the flow rate of auxiliary intake air, for instance is disposed at an air passage which bypasses a throttle valve, so that engine idle speed can be increased by opening the AAC value to a predetermined opening rate when the compressor is being actuated (at the time interval between t.sub.2 and t.sub.3 as shown in FIG. 6), as disclosed by NISSAN SERVICE WEEKLY, PB-71, No. 578, June, 1987.
In the above-mentioned prior-art engine idle speed control apparatus, the engine idle speed is increased whenever the compressor is being actuated, even under the condition that the thermal load applied to the compressor is relatively low and therefore the time intervals from when the compressor is deactuated to when actuated again are relatively long. Therefore, there exists a problem is that fuel consumption rate is high, because the engine idle speed is often increased, in spite of the fact that a sufficient cooling performance can be usually obtained.
In this connection, the above-mentioned NISSAN SERVICE WEEKLY discloses such an example that the idle speed is determined at a lower value (e.g. 600 rpm) when the after-evaporator temperature T.sub.INT is lower than a predetermined value (e.g. 8.degree. C.) and at a higher value (e.g. 875 rpm) when TINT is higher than the predetermined value (e.g. 8.degree. C.), whenever the evaporator is being actuated. In the prior-art control method as described above, however, since the engine idle speed is increased whenever the compressor is being actuated after the after-evaporator temperature T.sub.INT has risen beyond a predetermined value, irrespective of the thermal load applied to the compressor, there still exists a problem is that the fuel consumption rate is inevitably increased.
In addition, when the engine idle speed is increased under low thermal load, since the after-evaporator temperature drops quickly down to the evaporator freeze start temperature, the compressor is more frequently actuated and deactuated, thus resulting in another problem in that the durability of the compressor is deteriorated.